In a typical plant for the production of styrene and propylene oxide (S/PO), the following chemical reactions are conducted, using as feed reactants ethylene, benzene, oxygen, propylene and hydrogen:
1. Ethylene+benzene.fwdarw.ethylbenzene (EB)
2. EB+oxygen.fwdarw.ethylbenzene hydroperoxide (EBHP)
3. EBHP+propylene.fwdarw.propylene oxide(PO)+methyl phenyl carbinol (MPC)+methyl phenyl ketone (MPK)
4. MPK+hydrogen.fwdarw.MPC
5. MPC.fwdarw.styrene+water.
In the above exemplified reactions, organic by-products such as aliphatic and aromatic hydrocarbons, aldehydes, ketones, alcohols, phenols and organic acids are produced, in addition to reaction water. The by-products are separated from the main products, whereby clean water is used and the organic acids are neutralized with an aqueous solution of an alkali (such as sodium carbonate and/or sodium hydroxide). Additional water is introduced with the reaction gas (air) in the second above-exemplified reaction, and as steam in the fifth above-exemplified reaction. Thus on balance the plant as a whole uses clean water, and produces about 50% as much of wastewater which contains a wide variety of organic compounds, many of which are volatile, and organic salts.
The wastewater from an S/PO plant typically contains a total of from 1.0 to 3.5 wt % of non-salt organics and from 3.0 to 6.0 wt % of organic salts. In addition, it may contain from 0.0 to 2.0 wt % of sodium carbonate and sodium bicarbonate, and/or traces of sodium hydroxide depending on the alkali used in the neutralization step mentioned above.
The following is an exemplary composition of the waste-water from an S/PO plant (amounts given in % by weight):
______________________________________ methanol 0.1237 Sodium formiate 1.0100 acetaldehyde 0.0216 Sodium acetate 0.8400 ethanol 0.0233 Sodium propionate 0.2900 propionlaldehyde 0.0023 Sodium benzoate 1.9400 acetone 0.0281 propanol-2 0.0103 propanol-1 0.1317 benzene 0.0028 mono propylene glycol 1.2222 toluene 0.0016 methyl pentenal 0.0100 ethyl benzene 0.0036 styrene 0.0045 cumene 0.0007 benzaldehyde 0.2280 phenol 0.2700 benzyl alcohol 0.0026 methyl phenyl ketone 0.0175 methyl phenyl carbinol 0.1636 beta phenylethyl alcohol 0.0058 undefined 0.0604 ______________________________________
The input of clean water to such a plant can be up to several tens of thousands kg per hour, and the output of waste-water is about 50% higher than the input of clean water. The waste-water cannot be discharged without purification. The choice of a purification method for the wastewater of an S/PO plant is greatly restricted by practical, environmental and economic considerations. For example, biological degradation of the untreated wastewater is not feasible, due to the toxicity of several of its components (such as phenol) to the microorganisms involved in such degradation processes. Likewise, concentration by evaporation is not feasible due to evaporation of the volatile components, and distillation would involve prohibitive costs.
The currently employed method for purifying the wastewater stream of S/PO plants is basically a two-step treatment, the first step being oxidation with air at high temperature and pressure (known as the Wet Air Oxidation process, such as is constructed by Zimpro Inc., subsidiary of Sterling Drug Inc., Rothschild, Wis., USA), followed by bacterial degradation. This is a rather expensive procedure, and the need has existed for a long time to replace it by a more economic one.
It is already known to separate a pure substance, in particular water, from its dispersion by cooling the dispersion to below the freezing point of the intended pure substance (water) and separating the (ice) crystals. Various methods employing this principle, known as freeze-concentration or freeze-crystallization, are disclosed in, e.g., U.S. Pat. No. 3,283,522; U.S. Pat. No. 3,992,900; U.S. Pat. No. 4,004,886; U.S. Pat. No. 4,314,455; U.S. Pat. No. 4,457,769; U.S. Pat. No. 4,508,553 and U.S. Pat. No. 4,936,114. In several of these and other publications, the use of freeze-concentration has been proposed in general terms for desalinating sea water, and also for purifying unspecified wastewater. However, in practice freeze-concentration has been used predominantly in the food-industry, for the concentration of e.g. fruit juices, coffee extracts, milk and beer. It has not been used before for the large-scale treatment of wastewater from a chemical plant, in particular of the wastewater originating from an S/PO plant.
It has now been found that the principle of freeze-concentration can be successfully and economically employed for the separation of substantially pure water from the wastewater of an S/PO plant, whereby the wastewater can be concentrated at least two-fold, and preferably at least four-fold.